204 GEOLOGY OF THE BLACK HILLS. 



date of the uplift will be assembled ; finally, the age of the igneous masses 

 will be considered, and the section will close with a summary of the geo- 

 logical history of the district. 



1. — The form of the displacement. 



If the reader will open the geological atlas at the page which gives a 

 bird's-eye view of the Black Hills, he will obtain a clearer idea of the gen- 

 eral structure than it is possible to convey by words. 



Turning his attention first to the Archaean area just to the right of the 

 middle of the view, he will see that it presents a series of undulations 

 without any evident system. Around it there is a continuous cliff facing 

 toward it, so that it is on a grand scale walled in. The rock of this cliff is 

 the Carboniferous limestone. On the right hand side of the Archaean (the 

 eastern side) the Carboniferous has a considerable inclination which carries it 

 at once out of sight. At the left it has a very gentle dip, causing it to form 

 a plateau instead of a ridge, but at the farther edge of this plateau its dip 

 suddenly increases and carries it out of sight; so too at the north (or in 

 the distance) and at the south (or in the foreground). The Carboniferous 

 outcrop is completely surrounded by a line of high dip and its limits are 

 given thereby. With the aid of the view it is easy to understand that the 

 same limestone strata once extended completely across the oval area 

 marked out by the zone of high dip, and that there was no exposure what- 

 ever of the Archsean until it was denuded by the erosion and destruction 

 of a portion of the Carboniferous. The first step then toward restoring 

 the form which the rocks acquired by displacement but lost by erosion is 

 the replacement in imagination of the Carboniferous beds over the area 

 where now the Archaean only is exposed. 



Outside of the Carboniferous and completely encircling it is the Red 

 Valley, marking the outcrop of the soft clays of the Red Bed and Jura 

 formations, and outside of the Red Valley there is an encircling ridge 

 popularly known as "the foothills" and carrying the outcrop of the Dakota 

 sandstone. The sandstone and the soft beds beneath it conform to each 

 other and to the Carboniferous limestone and dip in the same direction, so 

 that the foothills consist everywhere of a monoclinal ridge, having a dip 



